An area scan camera captures a complete two-dimensional image in one moment and is the right choice for stationary or indexed parts. A line scan camera captures only a single image line at a time and assembles the full image from the relative motion between camera and object. It is superior for continuous material, for cylindrical parts, and whenever very high resolution is required over a long distance.
The fundamental difference
An area scan camera works like an ordinary photo camera: a sensor with, say, 2448 × 2048 pixels is exposed once and delivers a finished image. A line scan camera, by contrast, has only a single row of pixels, for example 4096 × 1. This line is read out in very rapid succession while the object moves underneath. Only the software assembles thousands of individual lines into a coherent image. This also means: without relative motion, a line scan camera produces no image at all.
Decision matrix
| Criterion | Area scan camera | Line scan camera |
|---|---|---|
| Part is stationary | Ideal | Not possible – motion is mandatory |
| Continuous material (web, belt, film) | Overlapping individual images needed, seams | Naturally suited, seamless image |
| Unrolling cylindrical parts | Several exposures, perspective error at the edges | Unwrapped surface in a single image |
| Very high resolution over a long distance | Multiple cameras or multiple exposures | One camera, length practically unlimited |
| Lighting effort | Moderate, illuminate the area evenly | High – very bright, precisely aligned line lighting |
| Integration effort | Low, a trigger is sufficient | Higher – encoder required for speed coupling |
| Total system cost | Lower | Higher (optics, lighting, encoder) |
When a line scan camera really pays off
The most common reason is not speed but the ratio of resolution to part length. An example: an 800 mm extruded profile is to be inspected across its full length for surface defects from 0.1 mm. Using the rule of thumb of three pixels per feature, roughly 24,000 pixels are required across the length – no single area scan camera provides that. A line scan camera with 4096 pixels across the direction of travel solves this, because the length is generated by the motion and is not limited by the sensor. An interactive calculator for the required resolution can be found in our article on flash detection in injection moulding.
Additional considerations with line scan cameras
- Encoder coupling: If transport speed varies, the image is compressed or stretched. A rotary encoder that synchronises line capture with the motion is therefore normally mandatory.
- Lighting intensity: Since each line is exposed only for an extremely short time, very bright line lights are required – typically far more powerful than for area scan cameras.
- Alignment: Lighting line, sensor line and direction of travel must be precisely aligned. Even slight rotation produces brightness gradients in the image.
- No snapshot: A line scan image is built up over time. Vibration therefore has a different effect than with an area scan camera and can distort the image rather than merely blur it.
The practical rule of thumb
- Part is stationary or indexed into position → area scan camera
- Continuous web, belt or film → line scan camera
- Cylinder or round part to be inspected over its full surface → line scan camera with rotary axis
- Very long part with fine defect size → line scan camera
- Several separate inspection features on a compact part → area scan camera
The camera type should follow from the inspection task, not the other way round. In practice, a large share of applications can be solved more economically with area scan cameras than initially assumed – and where a line scan camera is necessary, lighting usually decides between success and failure. THOP AG clarifies both on real parts as part of a feasibility study.